1. Field of the Invention
The present invention relates to an objective lens for a medical or industrial endoscope.
2. Description of Related Art
In a conventional objective lens for an endoscope, a first lens group on the object side is comprised of one negative lens (or a lens group having a negative power), and a lens group(s) having a positive power is provided behind the first lens group mainly to increase the angle of view, as in a retrofocus type lens arrangement. In such a retrofocus type lens system, light emitted from the first lens group is naturally divergent light, and the distance from the front surface of the first lens group to the image surface is usually long.
A medical endoscope that is inserted in a human body should be made as small as possible. The same is true for an industrial endoscope to facilitate the manipulation thereof. To this end, it is necessary to minimize the diameter and the whole length of the objective lens. However, in the conventional objective lens as mentioned above, there is a restriction to the minimization thereof.
Moreover, attempts have been made to realize more compact and smaller CCD's in the field of electronic endoscopes to thereby reduce the size of the endoscope. In general, in an electronic endoscope, an actual image circle produced by the objective lens needs to be larger than an actual image circle, because of a possible mechanical deviation of the optical axis of the objective lens from center of light receiving surface of the CCD. Since the deviation is substantially independent of the size of the CCD, an expected percentage deviation increases as the size of the CCD decreases, but is a function of the overall length of the objective lens.
For instance, in the case of a CCD having a diagonal length of 2.4 mm, the image circle of the objective lens is 2.4 mm. Assuming that the deviation is 0.1 mm, the image circle necessary for the objective lens is 2.6 mm. The ratio of 0.2 and 2.4 is 8.3% (0.2/2.4=8.3%). For a CCD whose diagonal length is 1.3 mm, the ratio is 15.4% (0.2/1.3=15.4%), this is a severe limitation on the objective lens.
Consequently, as the size of the CCD decreases, the image circle for the objective lens does not proportionally decrease as a relative percentage increases. The whole lens length must be as short as possible, in accordance with the minimization of the CCD.
In general, in an electronic endoscope, there is a glass cover in front of the front surface of the CCD. To prevent a physical interference between the glass cover and the objective lens, it is necessary for the objective lens to have a long back focal distance. Since the thickness of the glass cover has little or no relation to the size of the CCD, the back focal distance increases as the size of the CCD reduces (i.e., focal length of the objective lens decreases), which invites a difficulty upon design.
In summary, in an electronic endoscope having a small size CCD, there are incompatible requirements that the image circle must be relatively increased; the whole lens length must be reduced; and the back focal distance must be increased.